Antilock braking systems are commonly used in vehicles to improve braking effectiveness and, in particular, to prevent wheel lockup during panic braking. During braking, a conventional antilock braking system attempts to determine whether any or all of the tires of the vehicle have lost traction with the road. If traction has been lost, the antilock braking system suspends braking of the wheel until traction resumes. In this manner, wheel lockup during a panic stop is avoided. Also, braking and traction are improved on slick surfaces such as snow or ice.
To determine whether a tire has lost traction, the conventional antilock braking system first determines the angular rotation rate of each wheel The system then converts the angular rotation rate of each wheel to a corresponding effective forward speed. Next, the system estimates the actual speed of the vehicle by averaging the effective forward speed of the four wheels. The system then compares the estimated forward speed of the vehicle with the effective forward speeds of each individual wheel. A discrepancy indicates that traction has been lost.
Thus, the conventional antilock braking system must first estimate vehicle speed from a combination of individual wheel rotation rates. However, an estimate based on a combination of individual wheel rotation rates is inaccurate if one or more of the wheels is slipping, and grossly inaccurate if all four wheels simultaneously lock up.
The forward speed of the vehicle is preferably measured independently from the individual rotational rates of the tires. Unfortunately, an independent determination of the speed of the vehicle is difficult to achieve with conventional techniques. Heretofore, only sophisticated and expensive techniques were available for determining the speed of a vehicle independently from the angular speed of the wheels. Exemplary techniques include inertial accelerometer systems or doppler systems using radar or sonar reflections to measure vehicle speed relative to the ground. Such techniques, commonly employed only in aircraft, are expensive, complicated, and often unreliable.
In the conventional antilock braking system, if traction has been lost, the system momentarily releases the brake on the wheel that has been determined to be slipping. Ideally, the brake is released only for the minimum time sufficient to allow the slipping wheel to regain traction and begin rolling on the road surface. This optimum release time is estimated by using the coefficient of friction between the wheels and the road surface, with the coefficient of friction estimated from the angular deceleration of the wheel and from its inertia.
However, such estimates are unreliable, and the performance of the antilock braking system is degraded by an incorrect estimate of the coefficient of friction. For example, if the brakes are applied while a wheel is airborne, the system will incorrectly respond.
Further, the conventional antilock braking system requires an accurate determination of the forward velocity of the wheels based on the angular velocity and the tire radius. However, effective tire radius can vary significantly, thus rendering an inaccurate calculation of the forward velocity of the wheel.
The problems inherent in conventional antilock braking systems also occur in conventional vehicle traction control systems, because traction control systems also require a determination of the forward speed of the vehicle.
As can be appreciated, there is a need to provide an improved velocity sensor for a vehicle which does not rely on angular wheel velocities to determine the forward velocity of the vehicle, and does not require an estimation of the coefficient of friction between the tires and the ground.